1. Field of the Invention
The present invention relates to an exposure method and a projection exposure apparatus used in a lithography process in production of semiconductor elements, liquid crystal elements, or the like.
2. Related Background Art
There are two fundamental exposure methods conventionally used in such projection exposure apparatus. In one method, a photosensitive substrate such as semiconductor wafer, glass plate, etc. is exposed to light in a step and repeat method through a projection optical system having an exposure field which can include the whole pattern of mask or reticle. The other is a scan method in which a reticle and a photosensitive substrate are opposed to each other at the both sides of a projection optical system under illumination light of arch slit illuminates the reticle, and the reticle and the photosensitive substrate are relatively scanned for exposure under the illumination. Steppers employing the former step and repeat exposure method are leading apparatus recently used in the lithography process. The step and repeat exposure method has been improved in resolution, overlay accuracy, throughput, and so on, and became superior in these respects to aligners employing the latter exposure method. Therefore, it is considered that such steppers employing the step and repeat exposure method will be leading in lithography for a while.
It is proposed for the step and repeat exposure method that the photosensitive substrate and a best imaging plane of projection optical system be relatively moved in a direction of optical axis during exposure of one shot area in order to increase an apparent depth of focus of the projection optical system. This exposure method will be hereinafter referred to as a successive focussing exposure method. In this successive focussing exposure method, the moving amount in the optical axis direction is determined considering a real depth of focus of projection optical system and micro unevenness on the photosensitive substrate. The best imaging plane of projection optical system is arranged to be located between the top and the bottom of the unevenness on the photosensitive substrate surface during the movement.
Meanwhile, a novel scan exposure method achieving high resolution has been recently proposed as a step and scan method on pp 424-433, SPIE vol. 1088, xe2x80x9cOptical/Laser Microlithography IIxe2x80x9d, 1989. The step and scan method uses both a scan method, in which a reticle is one-dimensionally scanned and a photosensitive substrate is also one-dimensionally scanned at a speed synchronized with the reticle speed, and a step method, in which the photosensitive substrate is stepped in a direction perpendicular to the scan exposure direction.
FIG. 11 is a drawing to illustrate a concept of the step and scan method. In FIG. 11, a shot area of one chip or multiple chips is scanned for exposure with illumination light RIL of arch slit in the X-direction on a photosensitive substrate or wafer W. The wafer is stepped in the Y-direction. In FIG. 11 a broken line shows a sequence of exposure of step and scan as will be hereinafter referred to as S and S, so that the S and S exposure is carried out on shot areas SA1, SA2, . . . , SA6 in this order, and then on shot areas SA7, SA8 , . . . , SA12 arranged in the Y-direction in the center of the wafer. In the aligner of the S and S method as disclosed in the above-mentioned reference, an image of reticle pattern illuminated by the arch slit illumination light RIL is focussed on the wafer W through a one-quarter reduction projection optical system. Thus, a scan speed of reticle stage in the X-direction is controlled precisely to four times of that of wafer stage in the X-direction. The arch slit illumination light RIL is used because a demagnification system with a combination of refraction and reflection elements is employed as the projection optical system, and advantage is taken of various abberations being zero in a narrow annular region apart at a certain distance from the optical axis. An example of such reflection reduction projection system is disclosed in U.S. Pat. No. 4,747,678.
However, it is impossible that the successive focussing exposure method for the step and repeat method is applied to the step and scan method. In detail, the step and repeat method takes such a structure that the reticle/wafer and the illumination optical flux/exposure flux cannot be moved relative to each other in a direction perpendicular to the optical axis of the projection optical system, i.e., in a direction of wafer plane, upon exposure of one shot area. Therefore, a point of pattern in a transfer region on a reticle may be exposed at a plurality of focus points by relatively moving the wafer and the projection optical system in a direction of the optical axis upon exposure. In contrast, the step and scan method takes such a structure that the reticle/wafer and the illumination flux/exposure flux may be moved relative to each other in the direction perpendicular to the optical axis upon exposure of one shot area. In this structure, if the wafer and the projection optical system are moved relative to each other in the optical axis direction upon exposure, there would be mixed focussed parts and unfoccused parts on the wafer depending on positions in the transfer region on the reticle. Accordingly, if the same successive focussing method as in the step and repeat method is used for the S and S method, an increase in a depth of focus could not be expected, but degrading the resolution of image on the contrary.
It is, therefore, an object of the present invention to provide an exposure method and a projection exposure apparatus, applying a successive focussing method to a scanning exposure method, to obtain an increase in a depth of focus.
The object of the present invention, solving the above-described problem, can be achieved by a method for exposure in which a pattern IR formed in a transfer region on a mask R is subject to projection exposure through a projection optical system PL to be led onto an area to be exposure, or shot area, on a photosensitive substrate W, and the mask and the photosensitive substrate are at least one-dimensionally, relatively scanned with respect to a projection field IF of the projection optical system PL: comprising, limiting a width of the area of pattern image projection on the photosensitive substrate W through the projection optical system PL to an approximately constant value in a direction of one-dimensional scan; and inclining a local surface on the photosensitive substrate W on which the pattern image is formed, relative to a best focal plane BF of the projection optical system PL in the direction of one-dimensional scan.
Also, the object of the present invention can be achieved by a projection exposure apparatus comprising: a projection optical system PL for projecting a pattern IR formed in a transfer region on a mask R, onto an area to be exposed, or shot area, on a photosensitive substrate W; a mask stage 14 for one-dimensionally moving the mask R over a region beyond a width of the transfer region in a direction of movement; a substrate stage 17, 18 for one-dimensionally moving the photosensitive substrate W in the direction of one-dimensional movement of the mask stage 14 at a speed synchronized with a movement speed of the mask stage 14; an illumination system 1-13 for illuminating the mask R with an illumination flux for exposure, having a shape between a rectangle and a slit within the projection field IF of the projection optical system PL and having an approximately constant width in the direction of one-dimensional movement; a substrate holder 16 for holding the photosensitive substrate W on the substrate stage 17, 18 with a predetermined inclination angle with respect to the direction of one-dimensional movement of illuminated area formed by the illumination flux on the photosensitive substrate through the mask R and the projection optical system PL; a holder drive system 21 for moving the substrate holder 16 in a direction of optical axis AX of the projection optical system such that a central part of the illuminated area on the photosensitive substrate W is located near a best focal plane of the projection optical system PL; and a control system 54 for controlling the holder drive system 21 to maintain an imaging condition of pattern image of the mask R on the photosensitive substrate W with correspondence to a position in the illuminated area in the direction of one-dimensional movement while scan exposure of pattern of the mask is effected on the area to be exposed.
According to the present invention, in a projection exposure apparatus of scanning exposure method, the substrate holder is arranged to hold the photosensitive substrate with the predetermined inclination to the one-dimensional scan direction of illumination area of illumination flux, so that the best focal plane of projection optical system and the illuminated area on the photosensitive substrate may be inclined relative to each other. Further, the holder drive means is provided in translate the substrate holder in the optical axis direction of projection optical system, such that the central part in the illuminated area on the photosensitive substrate is located at or near the best focal plane of the projection optical system. This arrangement allows continuous or discrete change of focussing of pattern image on the reticle for scan exposure. In other words, the depth of focus may be effectively increased.